1. Field of the Invention
The present invention relates to a three-dimensional acceleration sensor, and particularly to a technique for improving sensitivity and impact resistance of an acceleration sensor manufactured by a micro-fabrication process.
2. Description of the Related Art
A conventional three-dimensional acceleration sensor generally comprises a weight portion, a base portion provided around the weight portion, and beam portions each of which movably connects the weight portion and the base portion and is provided with a strain detection part, all of which are formed in a laminated substrate. The acceleration sensor detects the displacement of the weight portion through the strain detection part of each beam portion to thereby determine acceleration. Although a patent document 1 (Japanese Laid-Open Patent No. 2002-296293) has disclosed that the sensitivity of an acceleration sensor is improved by increasing the mass of a weight portion. The conventional three-dimensional acceleration sensor described in the patent document 1 discloses that the mass of the weight portion is increased to improve the sensitivity of the acceleration sensor. Since, however, the acceleration sensor per se is also brought into less size with advances in its miniaturization, an increase in the mass of the weight portion is becoming difficult correspondingly. Thus, a problem arose in that the improvement in sensitivity by the mass of the weight portion was becoming difficult. Also a problem arose in that when much acceleration was applied in a vertical direction with the increase in the mass of the weight portion, the amount of displacement of the weight portion exceeded the allowable amount of displacement of a beam portion and varied, thereby causing breakage of the acceleration sensor. These got in the way of improving the sensitivity of the acceleration sensor and bringing it into less size. In addition, the patent document 1 does not disclose a stopper portion and a detailed manufacturing process for the acceleration sensor.
On the other hand, a patent document 2 (Japanese Laid-Open Patent No. 2004-198243) discloses that the thicknesses of beam portions and stopper portions in the vertical direction are identical. The conventional acceleration sensor described in the patent document 2 does not take up a problem about means for improving the sensitivity while having high impact resistance because the stopper portions and the beam portions are identical in thickness.